The invention relates to a system comprising a plurality of data-receiving stations distributed over a network.
In general, a database system comprises a central station in which data is stored on a permanent basis, generally on a magnetic disk referred to as a "hard disk". In certain applications, the data comes from decentralized stations.
Each station receives data that must be backed up in temporary or permanent manner on another station in order to provide against failures at the station receiving the data.
Backup takes place in read/write memory and/or in non-volatile memory and at least (or at most) one other station. This backup can also be performed in non-volatile memory at the station concerned and/or on the hard disk of the central station. The type of backup depends on the nature of the data.
Thus, in a telecommunications system, the central station contains data that is modified rarely and that is used frequently, which data is backed up on hard disks. By way of example, this data constitutes subscriber descriptions and the configuration of the network. This data backed up on hard disks is referred to as "semi-permanent" data. Also, other data referred to as "product" data, such as billing, is backed up in general in the read/write memory and/or the hard disk of several other stations. Finally, so-called "context" data such as memorizing the location of a mobile telephone, or the connections that have been set up to establish calls, are backed up in read/write memory and/or in non-volatile memory in at least one other station.
Whatever the location in which the data is backed up, backup can be performed in two modes: a synchronous mode, and an asynchronous mode.
Synchronous backup consists in making a replica or copy of the data simultaneously or quasi-simultaneously with the data being put into memory in the originating station. This type of backup provides a high degree of security since it takes place in real time and is therefore unaffected by any incidents that might occur subsequently. Nevertheless, it is burdensome since it requires the use of a call while the data is being inserted. Also, the length of time taken to put the data into memory at the originating station is determined by the duration of the call, and that is generally significantly longer than the time required merely for putting data into memory.
Asynchronous type backup consists in making modifications on the original, i.e. at the data-issuing station, independently of transmitting data to the copies or replicas. Transmission to other stations takes place in deferred manner. It is performed either when a communications system becomes available, or else in periodic manner, at previously selected moments.
With asynchronous backup, updating takes place locally more quickly than when backup is performed synchronously, since the original can be updated without being subjected to the time required for transmitting the data to another station. In contrast, such backup is less secure since between the data being updated and the data being transmitted, i.e. backed up, the update may be lost because of an incident occurring at the station concerned.